Bring atomic flag fallback up to snuff

Hi,

As Daniel pointed out in:
https://postgr.es/m/FB948276-7B32-4B77-83E6-D00167F8EEB4@yesql.se the
pg_atomic_flag fallback implementation is broken. That has gone
unnoticed because the fallback implementation wasn't testable until now:
- /* ---
- * Can't run the test under the semaphore emulation, it doesn't handle
- * checking two edge cases well:
- * - pg_atomic_unlocked_test_flag() always returns true
- * - locking a already locked flag blocks
- * it seems better to not test the semaphore fallback here, than weaken
- * the checks for the other cases. The semaphore code will be the same
- * everywhere, whereas the efficient implementations wont.
- * ---
- */
and flags weren't used until recently.

The attached fixes the bug and removes the edge-cases by storing a value
separate from the semaphore. I should have done that from the start.
This is an ABI break, but given the fallback didn't work at all, I don't
think that's a problem for backporting.

Fix attached. Comments?

Greetings,

Andres Freund